Smith & Nephew Plc v Convatec Technologies Inc

A numerical range in a patent claim is construed purposively, through the eyes of the skilled person, using the common general knowledge and the specification as a whole. The same construction governs infringement and validity.

Rounding cannot alter the prior-art disclosure or the alleged infringement. The skilled person may nevertheless understand that the patentee expressed the claim limits to a limited degree of precision. That question depends on the whole context, including the decimal places or significant figures used.

Where a claim specified concentrations of “between 1% and 25%” as whole numbers, while the specification used differing degrees of precision, the range covered values at least 0.5% but below 25.5%.

ConvaTec owned a patent for a process used to make light-stabilised, silverised antimicrobial material. Claim 1 required the silver-binding agent to be present at a concentration “between 1% and 25%” of the total treatment volume.

Smith & Nephew sought a declaration that its modified manufacturing process did not infringe. ConvaTec counterclaimed concerning that process and an original process used in four experiments. Birss J held in [2013] EWHC 3955 (Pat) that the claimed range covered concentrations at least 0.95% but below 25.5%. He found that the modified process did not infringe but that the original process did.

Both sides appealed. The central issue was whether the range began at exactly 1%, 0.95% under a significant-figures approach, or 0.5% under a whole-number approach.

1. ConvaTec’s appeal allowed; Smith & Nephew’s appeal dismissed. The judge’s construction was unsustainable. Claim 1 covered binding-agent concentrations at least 0.5% but below 25.5%. The alternative issue about the empirical calculation of carryover therefore did not require determination. Kitchin LJ delivered the principal judgment. Briggs and Christopher Clarke LJJ agreed.

2. Patent claims containing numerical ranges are construed by the same principles as other claims. The objective question is what the skilled person, equipped with the common general knowledge, would understand the patentee to mean in the context of the specification. The claims demarcate the monopoly, and an apparently deliberate numerical limitation cannot ordinarily be treated merely as a general guide. The construction must be identical for infringement and validity.

3. Rounding cannot be used to change the prior-art disclosure or modify the alleged infringement. The skilled person may, however, understand from the common general knowledge and the specification that the patentee expressed a claimed number only to a limited degree of precision. A comparison value may then need to be stated to the same precision. Whether this is appropriate depends on all the circumstances, including the number of decimal places or significant figures used.

4. The claim limits were not exact values. The specification showed that the patentee knew how to use zero, one and two decimal places, yet chose whole numbers in the claim. It also taught that the invention could operate over a much wider concentration range. The word “between” identified the outer limits but did not determine their precision.

5. The whole-number approach accorded with the way the limits were written. The significant-figures approach produced anomalous and asymmetric results which bore no relationship to practical random error. Thus 1% embraced values at least 0.5% but below 1.5%, while 25% embraced values at least 24.5% but below 25.5%. The claimed range as a whole therefore extended from 0.5% inclusive to 25.5% exclusive.

1. Court of Appeal (Civil Division): In [2015] EWCA Civ 607, allowed ConvaTec’s appeal and dismissed Smith & Nephew’s appeal. It replaced the judge’s construction with a range of at least 0.5% but below 25.5%.
2. High Court, Patents Court: In [2013] EWHC 3955 (Pat), Birss J construed the range as at least 0.95% but below 25.5%. He held that the modified process did not infringe but that the original process did.